Tremendous strides have been made in the field of analytical chemistry in recent years. Conventional "wet" chemical analysis which is slow and laborious suffers from a large number of variables that can be present in the physical procedures involved, and has been largely replaced by more sophisticated techniques. One such technique involves the spectral analysis of particles that are introduced directly into a plasma. The high temperature and high energy conditions prevelant in the plasma cause molecules and atoms to emit their characteristic spectra which can be readily identified.
Plasmas of excited gases are generated by a variety of means including chemical flames, arc- or spark-discharges, inductively coupled radio frequency, and microwave-sustained gas discharges using excitation gases such as argon, helium, nitrogen, or oxygen.
While high energy plasmas are used to excite atoms and molecules to an emissive condition, a specific advantage of a helium plasma is that it has sufficient energy to generate principally atomic emission spectra for the elements present in the sample. A helium plasma device known as a microwave-sustained emission detector has recently been employed to great advantage for qualitative and quantitative gas chromatographic analysis.
Recent publications on microwave plasma spectroscopy include "Microwave Induced Plasma" by J. W. Carnahan in American Laboratory, 15, 31-36 (1983), "Determination of Halogenated Organic Compounds in Water by Gas Chromotagraphy/Atmospheric Pressure Helium Microwave-Induced Plasma Emission Spectrometry with a Heated Discharge Tube for Pyrolysis" by K. Chiba and H. Haraguchi in Anal. Chem., 55, 1504-1508 (1983), and "Microwave Induced Electrical Discharge Detectors for Gas Chromatography" by T. H. Risby and Y. Talmi in CRC Critical Reviews in Analytical Chemistry, B. Campbell ed., CRC Press, Inc., Boca Raton, FL, 33431, 1983, 231-265. A commercially available microwave plasma detector (MPD) is a combination of a microwave plasma and a grating spectrometer and limits of measurement and examples of application are described by K. S. Brenner in J. Chromat., 167, 365-380 (1978).
Elemental analysis of volatilized substances can be accomplished using a microwave emission detector (MED), which is also known in the art as a microwave plasma detector (MPD), a microwave-induced plasma detector (MIP), and a helium microwave-sustained emission detector (He MED) where helium is the plasma gas.
Elemental analyses using a microwave plasma detector wherein volatile particulate or gaseous samples are transported in a carrier gas to a plasma are disclosed in U.S. Pat. Nos. 4,225,235, 3,923,398, 3,843,257 and British Pat. No. 1,368,810. U.S. Pat. No. 4,136,951 discloses use of a dual plasma system and here also a vaporized sample is transported in a carrier gas from a pyrolysis plasma to a secondary plasma which is spectrally analyzed.
U.S. Pat. No. 4,330,295 discloses the coating of a polymer solution onto a quartz capillary tube or use of an organic sample in a solid form such as a film, filament, or small bead, subjecting the sample to a microwave discharge to decompose and volatilize it, then transporting the decomposed fragments by means of a carrier gas for subsequent separations and determinations. In this reference, the plasma and plasma-containing energized fragments were not subjected to direct analysis; instead they were transported to another analytical device for analysis.
A key weakness in state-of-the-art MED units is the method of sample introduction. When samples are easily volatilized and precautions are taken to minimize conduit (wall) interactions, atomic emission of the sample gives a quantitative representation of the elements or elemental ratios present in the sample. For example, volatile components, separated from a gas chromatograph, can be transported from the gas chromatograph to the excitation plasma of an MED unit by means of a heated conduit, such as glass-lined stainless steel capillary tubing, and present little if any problem. However, volatile samples comprise only a small portion of the many materials subject to analysis using an MED. A widely utilized means for volatilizing organic compounds involves pyrolysis or fragmentation to smaller more volatile entities, which are then transported by means of a carrier gas through heated conduits to the excitation plasma.
Use of a carrier gas to transport particles to and from the plasma results in certain shortcomings. Particulate fragments frequently recombine or condense on the conduit walls or are absorbed on the walls of the pyrolysis chamber and transporting conduit. The results in loss of material and "ghosting", that is, materials sorbed onto conduits (commonly referred to as transfer lines), interface surfaces, sample probes, valves and the like may be introduced into the MED during subsequent sample analysis and can lead to spurious, erroneous analytical results. This loss of part of the sample and the difficulty in maintaining reproducible pyrolysis temperatures and other conditions make it difficult to obtain quantitative and reproducible results. It is virtually impossible to completely volatilize most organic polymers by pyrolysis without losing fractions to the walls of the pyrolysis apparatus and to the transporting conduit.
A technique in common usage in the background art is to aspirate and atomize or nebulize non-volatile material into a plasma. When materials are introduced to a plasma by apsiration, nebulization, atomization, and the like, more subtle deficiencies are noted. For example, when an aerosol reaches a plasma, the aerosol may absorb energy from that plasma and as a result, may seriously affect plasma operating conditions and performance. In an extreme case, sample introduction may overload the plasma sufficiently to extinguish it. None of these devices or techniques utilizing solvent aerosols for the introduction of non-volatile liquids or solids can be used with a helium MED since the elements in the solvent constitute an interference with the determination of the elements present in the sample.
To overcome the disadvantages of transporting particles and vaporized samples to a plasma in a carrier gas, some efforts have been directed to sample introduction directly into a plasma, as for example in U.S. Pat. No. 3,467,471 and Re. 29,304. However, the shortcomings such as incomplete fragmentation, irreversible absorption of fragments on the conduit, and the possibility that too much sample, especially in powder form, may extinguish the plasma, are not overcome.